1. Field of the Invention
This invention relates generally to a powertrain for a hybrid electric vehicle (HEV). More particularly, it pertains to the control of transitions between series drive and parallel drive operation of the powertrain.
2. Description of the Prior Art
The powertrain for hybrid electric vehicle may include two electric machines in combination with an engine and transmission to operate in at least two operating modes, series and parallel drive, sometimes called a dual-drive hybrid-electric powertrain configuration. The first electric machine is mechanically coupled between the engine and transmission on the front axle in order to provide starter/generator capability. The second electric machine is connected to the rear axle in order to provide additional propulsion capability in either an electric or hybrid drive mode, resulting in two independently driven axles. The electric machines are powered by a high-voltage battery using inverters.
This powertrain configuration provides great flexibility for operating the powertrain in various modes, such as electric mode, series mode, and parallel or split mode to satisfy the driver's demand and achieve better fuel efficiency without compromising other vehicle performance attributes.
Given the architectural complexity and the operational flexibility of this powertrain, it is essential to have a highly coordinated vehicle control system to perform the blending of torque, speed, and power from multiple power sources in addition to managing transmission, engine and electric machine subsystem control.
A need exists in the industry for a control method that produces transition between series drive mode and parallel or split drive mode that takes into account various sources of information about the driveline and state of the electrical drive components.